Caller identification (CID) service is the generic name for a service provided by the telephone companies (i.e., Stored Program Controlled Switching systems; SPCSs) to deliver information such as the caller's telephone number and/or name to a telephone set (i.e., Customer Premises Equipment; CPE) of the called subscriber at the beginning of a call. A variant of CID, Caller Identification on Call Waiting (CIDCW), delivers this information about an incoming caller while the called subscriber is already engaged in a phone call.
The caller identity information can be used in many ways. A few examples include tracking who has called over a specified period of time, accessing data base information on the calling party, tracing malicious callers, storing number in memory for quick redialing, and blocking unwanted calls.
In most countries, the caller identity data stream is transmitted in 1200 baud Bell 202 standard or CCITT V.23 FSK (Frequency Shift Keying) format. The telephone or adjust box demodulates the FSK signal and displays the caller's number and/or name on an LCD (Liquid Crystal Display). In FIG. 1, there is shown a format of the transmitted data stream for the CID service.
Referring to FIG. 1, the transmitted data stream includes a channel seizure signal 21 which serves to notify a CPE that the caller identity data packet (or CID data message) will be transmitted followed by a mark signal 22 containing a train of "1" bits. This mark signal 22 is used to identify the head of a data message. The data stream further includes CID data packet 20 which is composed of ASCII codes of which is framed by a start bit and a stop bit. The data packet 20 contains the CID information on telephone number, name, month, date, hour, minute and so on. The size of data packet 20 is 144 bits in the U.S.A. and is about 250 bits in Canada. The data stream finally includes a checksum word signal 23 which is transmitted after the data packet 20. The checksum signal 23 is used to ensure that the CPE has received the data packet correctly. That is, error detection is provided by the use of the checksum word 23.
FIG. 2A is a diagram illustrating the caller information reception process of a telephone set 10 (i.e., CPE) in an on-hook state. In this state, the CID data is transmitted to a called subscriber during a 4 second pause interval between first and second ring signals RING#1 and RING#2. The ring signals are used to ring a bell of the telephone set (i.e., CPE) 10 at the called party and are each continuous for about two seconds, as is well known.
FIG. 2B is a diagram illustrating the caller information reception process of a telephone set in an off-hook state. In this state, SPCS 12 applies a CPE alerting signal ALERT of 2130 Hz and 2750 Hz dual tone to the CPE (i.e., telephone set ) 10 of the called subscriber for 80 msec I/-5 msec. This signal is intended to alert the CPE to prepare for the incoming CID data. Within about 100 msec after detecting the CPE alerting signal ALERT, the CPE 10 should reply to the SPCS 12 with an acknowledgment signal ACK. Once the SPCS 12 has detected the acknowledgment signal ACK, it transmits the CID data to the CPE 10 via 1200 baud Bell 202 format FSK signal. But, in this case, the channel seizure signal is not transmitted and only the mark signal is transmitted to the CPE only for 66.7 msec.
FIG. 3 is a block diagram showing an example of an analog telephone set having CID function only in the on-hook state. Referring to FIG. 3, the telephone set includes a CID receiver 30, two switches 40 and 44, AC coupler 42, and telephone circuitry 46. Further, the CID receiver 30 includes a ring detector 32, a demodulator 34 for performing the demodulation of the received FSK signals, an energy estimator 36 for estimating the energy of the received signals, and a comparator 38 for converting the demodulated signals to a serial bit stream.
The ring detector 32 serves to detect whether the first ring signal is applied, and to close the switch 40 when the first ring signal is detected. The AC coupler 42 performs AC-coupling for the CID data signal received via the telephone line. This circuit 42 allows the telephone set to remain in the on-hook state substantially, regardless of the operation state of the CID receiver 30. The AC-coupled signal is provided to the demodulator 34. The ring detector 32 then causes the switch 40 to be open before the second ring signal is received. When the called subscriber lifts the handset of the telephone set after he/she identifies the caller's identity, the switch 44 becomes closed and the telephone circuitry 46 thus goes to the off-hook state.
According to this technique, it is easy for a CID receiver to receive the CID data in the on-hook state because the receiver 30 is equipped with the ring detector 32.
However, in order to receive the data in the off-hook state, the receiver 30 needs a tone detector for detecting the CPE alerting signal transmitted from the SPCS and a tone generator for generating the acknowledgment signal in response to the CPE alerting signal. In addition, there is a drawback that it is fairly difficult to design the demodulator and comparator suitable for noise characteristics and relatively high transmission speed in analog techniques. Additional external logic circuitry is further required for processing the channel seizure signal, mark signal, start bit and stop bit since the final output of the CID receiver is the serial bit stream.